The locations of underground utilities such as power, water, sanitary, telephone, gas, cable television (CATV), and non-potable may be marked with devices known as passive underground electronic markers. Typically, each marker contains one or more Inductor-Capacitor (LC) circuits, or equivalent circuitry, each of which is tuned to a unique frequency associated with a particular utility type. In that regard, each circuit typically includes a coil of wire that responds to reception of a signal transmitted at a particular frequency. The markers are buried in a trench with the utility structure to be located during surveying, maintenance operations, or other activities requiring precise location of the utility.
In order to locate a buried marker, an operator moves across the surface of the ground with a locator, which typically includes both a transmitter and a receiver. The transmitter sends out an electromagnetic signal tuned to the unique frequency of the marker associated with a particular utility type. Upon receiving the transmitted signal, the marker resonates and thus produces an electromagnetic response to the transmitted signal. This response is received by the receiver and converted to a signal that is detectable by the operator. The operator may mark the pinpointed location on the ground using chalk, paint, or other surface marking indicia, and then move on to find the next marker buried along the utility structure. Examples of passive marker and locator systems are disclosed U.S. Pat. Nos. 5,699,048, 6,097,293, 6,380,857 and 6,388,575, the disclosures of which are incorporated herein by reference.
A common problem associated with conventional utility marker location technologies arises due to the fact that, often, more than one utility type will be in the same vicinity. Close proximity of various utility types generally requires that the location process be repeated at the respective unique frequency for each respective marker type. Conventional marker locators are typically designed to detect only one or two marker types. The presence of more than one or two utilities in close proximity will require that the location process be repeated for every marker type.
Conventional locator apparatus that can be configured to locate multiple marker types must, in any event, be switched manually between frequencies appropriate for each respective marker type. If the user is looking for all marker types, the user has to remember which frequencies have been checked and which frequencies remain untested. The manual switching between marker types introduces the possibility of user error, and allows marker types to be skipped or forgotten. Specifically, because the search pattern has to be repeated for each marker type, a possibility always exists that the operator will become complacent and not search for each type thoroughly.